The vast regulatory potential of the adenylate and guanylate energy systems allow for interpretation for possible regulation of multiple metabolic pathways, some of which are essential for maintenance of the differentiated state. We have observed three different enzymatic forms of adenylate kinase in animal tissue. Each form has unique kinetic and physical properties, and differs in its potential ability to regulate or aid in the maintenance of a homestatic internal cellular environment. We propose to examine the adenylate and guanylate energy charge systems in normal and neoplastic tissues in regard to modulation by intracellular metabolites, free fatty esters, prostaglandins, and new derivatives of the purine nucleotides. We have observed increased adenylate and guanylate energy charges in fast growing tumors. Alterations in metabolic controls of this kind could be an essential requirement for the neoplastic state.